[sdiy] Voltage Feedback Resistors and Circuit Stability

[Bernard Arthur Hutchins, Jr]

First, the question was about : [sdiy] Voltage Feedback Resistors and Circuit Stability<https://synth-diy.org/pipermail/synth-diy/2020-November/174435.html>.   “Feedback resistance“ makes almost no sense.   It is a feedback RATIO that maters, as a design objective (e.g., setting gain) and in any (if any at all) op-amp stability issues.  Typically, this ratio is set by a series resistive voltage divider Vout/Vin=R2/(R1+R2) where R1 is connected to Vin, and R2 goes (usually) to ground.  A ratio of 1/11 (0.090909…) is obtained by R2 = R1/10.  R2 might be 10k with R1=100k, or R2 might be 1k with R1 = 10k, etc.  If Vin is the output of an op-amp, good practice suggests that the SUM of R1 and R2 should be at least several k (for the op-amp to drive) and less than about a meg (to avoid stray signal pickup). ELSE which values do you have the most of.

All this freedom goes out the door IF THERE ARE CAPACITORS IN THE DIVIDER LOOP!  In this case, if you scale the resistors by a factor B, you must scale the capacitors by 1/B. I can’t recall a familiar example of this in a feedback case, but in an input attenuator case, one is very familiar: the case of an R1 =100k, R2= 220 ohm attenuator into our original OTA integrators  (S-V VCFs).  Originally no capacitors were used.  Then we started to use shunting “phase-lead” capacitors across the 100k R1.  The needed shunting capacitor was an inconveniently small (rare, and comparable to stray) 3pf or so. This is why we changed R1 down to 10k, R2 down to 22 ohms, and C up to a more agreeable 30 pfd.

Finally, keep in mind that “instability, in general, is intuitively associate with high gain (like positive feedback in PA systems).  In the case of linear op-amp applications, NEGATIVE feedback is used to restrain the extremely large gain of otherwise open-loop devices.  Low gain circuits have MORE (presumed negative) feedback.  To the extent that actual feedback slides slightly less negative, a high-frequency oscillation may kick in.  Maximum (negative) feedback (100%) is at unity gain – hence the near universal unity-gain internal compensation.  Such an op-amp “follower” may oscillate if asked to drive a long scope cable (capacitor) for example (phase shift inside the  loop) while being perfectly well-behaved if the gain is perhaps 4!                                                                                                                                                                              - Bernie

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